Paint coated sheet material with adhesion promoting composition

ABSTRACT

A protective and decorative, stretchable, thermoplastic sheet material for bonding by thermoforming to automobile body panels and other articles which comprises a polyester carrier film coated with an aqueous polyurethane paint composition. To aid in the adhesion of the paint, the carrier film is coated with a tie coat composition comprising a self-crosslinking, aqueous polyurethane dispersion.

FIELD OF THE INVENTION

This invention relates to a paint-coated sheet material and, moreparticularly, to such a sheet material which is coated with acomposition for improving the adhesion of a paint layer to astretchable, thermoplastic carrier film.

BACKGROUND

The patent application of G. Reafler, Ser. No. 116,426 filed Nov. 3,1987 describes a new paint-coated, thermoformable sheet material ofexceptional high quality. It is useful for applying protective anddecorative surfaces to substrates such as automobile panels, householdappliances and the like. The new paint-coated sheet materials affordsavings in paints as compared with conventional spray paintingprocesses. Moreover, air pollution from evaporating paint solvents canbe reduced.

Through the use of laminar flow coating techniques as described in theReafler patent application, sheet materials are formed which haveoutstanding surface properties such as gloss and distinctness of image.A preferred species of the new protective and decorative sheet materialis formed by coating an aqueous polyurethane paint on a stretchable,thermoplastic polyester carrier film. To improve the adhesion of theaqueous polyurethane paint to the polyester carrier films, an adhesionpromoting coating or, as it is also called, a tie layer, is coated onthe carrier film before the paint is applied.

Although previous tie layers have improved adhesion, very strongadhesion is required of sheet materials to be used as exterior coveringsfor automotive panels and other articles. It is difficult to achievestrong adhesion of a water-based polyurethane paint composition to astretchable, hydrophobic polyester carrier film even though a tie coatis used. The exposure of such materials to heat and humidity furtheraggravates the problem. Hence, a sheet material with improved adhesionof the aqueous polyurethane paint to the polyester carrier film has beenneeded.

SUMMARY OF THE INVENTION

The present invention provides a protective and decorative sheetmaterial of the type described having improved adhesion of the aqueouspolyurethane paint coating to the polyester carrier film. Theimprovement is achieved by using as a tie layer a particularpolyurethane composition.

The sheet material of the invention is a protective and decorative sheetmaterial for use in a thermoforming process in which it is stretched andbonded to a three dimensional substrate. It comprises a flexible andstretchable thermoplastic polyester carrier film, a stretchable aqueouspolyurethane paint layer, a stretchable transparent polyurethane topcoatlayer and, disposed between the carrier film and the paint layer, a thintie layer formed by coating a self-crosslinking aqueous polyurethaneanionic colloidal dispersion, the polyurethane being derived from apolyurethane prepolymer of which the chain is extended with an aliphaticpolyamine chain extender and is end-capped with N-methylol hydrazidegroups, the prepolymer being derived from an aliphatic diisocyanate, analiphatic polyester polyol and having pendant water-dispersingcarboxylic salt groups in the polymer chain.

THE DRAWING

The sole FIGURE of the drawing is an enlarged, diagrammatic crosssection of a sheet material of the invention.

DETAILED DESCRIPTION

The sheet material of the invention is a flexible and stretchable sheetmaterial of the class described in the Reafler patent application Ser.No. 116,426 of Nov. 3, 1987. This sheet material is a paint-coated filmwhich is applied by thermoforming to the surfaces of automobile panelsor other articles to serve as a protective and decorative covering. Theterm thermoforming is used herein to include the vacuum forming andbonding of a heated thermoplastic film to a metal, plastic or othersubstrate as well as the "in-mold" bonding of such a film to plasticparts in a mold.

The sheet material includes a flexible and stretchable carrier film andpolymeric coatings such as paint layers and clear topcoats, which haveheat softening and tensile elongation properties that are compatiblewith those of the carrier film so that the sheet material can bestretched without crazing or delaminating the coated layers.

The drawing is an enlarged, diagrammatic cross-section, not to scale, ofa particular sheet material of the invention. It includes a flexible andstretchable thermoformable polyester carrier film 11. The carrier filmhas a substantially uniform thickness which can be in the range fromabout 0.05 to 0.40 millimeters. Preferably, the thickness is from about0.16 to 0.32 millimeters. Coated on the carrier film is a thin tie layer12, the composition of which will be described in detail hereinafter.Coated over the tie layer 12 is the basecoat or paint layer 13, thethickness of which can be in the range from about 0.012 to 0.08millimeters. Over the paint coat 13 is a transparent crosslinkedpolyurethane topcoat 14 which normally is substantially thicker than thebasecoat, e.g., having a thickness in the range from about 0.02 to 0.25millimeters. Optionally, the back side of carrier film 11 is coated withan adhesive layer 15 to facilitate bonding of the sheet material tosubstrates. Preferably, this is a pressure-sensitive adhesive layer inwhich case a protective release sheet 16 of polyester film or the likeis temporarily bonded to the adhesive layer to protect it prior tothermoforming. In the "in-mold" method of thermoforming an adhesiveusually is not necessary because bonding is achieved by fusion of thecarrier film to the molded plastic article.

The polyester carrier films used in the sheet materials of the presentinvention are formed by melt extruding thermoplastic immiscible blendsof a relatively rigid, linear polyester and a more flexible copolymer.The term polyester carrier film as used herein is intended to includesuch blends.

A preferred linear polyester for such blends is a copolymer ofterephthalic acid, 1,4-cyclohexanedimethanol and a lower alkyleneglycol. The more flexible copolymer can be a polymer such as arubber-modified, styrene-maleic anhydride copolymer or apoly(etherester) derived from 1,4-cyclohexanedicarboxylic acid.

A preferred carrier film composition is an immiscible blend of acopolyester based on poly(1,4-cyclohexylenedimethylene terephthalate)and a more flexible rubber-modified styrene-maleic anhydride copolymer,as described in the patent to Weemes et al, U.S. Pat. No. 4,582,876dated Apr. 15, 1986. More particularly, the blend contains (a) 70 to 30weight percent copolyester consisting of repeating units fromterephthalic acid, repeating units from 1,4-cyclohexanedimethanolpresent in the amount of 25 to 90 mole % and repeating units from asecond glycol having 2 to 6 carbon atoms present in the amount of 75 to10 mol %, wherein the total mol % of glycol is equal to 100 mol %, and(b) 30 to 70 weight percent thermoplastic rubber-modified vinyl aromaticpolymer composition comprising (1) from 60 to 93% by weight of anonequimolar copolymer of 70 to 98% by weight based on a copolymer of avinyl aromatic monomer and from 2 to 30% by weight based on a copolymerof an unsaturated dicarboxylic acid moiety copolymerizable therewith,and (2) from 7 to 40% by weight of at least two differing rubberyadditives which were present during the copolymerization of the monomersof component (1), wherein the rubbery additives comprise from 5 to 20%by weight based on the total composition of at least one high vinylaromatic conjugated diene copolymer wherein the at least one highconjugated diene-vinyl aromatic copolymer comprises from about 60 toabout 98% by weight of the conjugated diene. At least one high vinylaromatic conjugated diene copolymer preferably is a block copolymerwhich comprises a greater amount of vinyl aromatic than conjugated dienemonomer. The composition can include fillers, UV absorbers,plasticizers, colorants, antioxidants, etc. known to be useful inpolymeric films.

Another preferred composition for carrier films is described in thecopending application of Seymour, Ser. No. 151,727 filed Feb. 3, 1988.This composition forms a very flexible carrier film which is especiallyuseful for applying coverings to plastic articles by the "in-mold"process. The composition is an immiscible blend of a flexiblepoly(etherester) with a more rigid linear polyester of terephthalicacid. More specifically, the blends comprise

(A) about 98-2 weight % of a flexible poly(etherester) having an I.V. ofabout 0.8-1.5 and recurring units from

(1) a dicarboxylic acid component consisting essentially of1,4-cyclohexanedicarboxylic acid having a trans isomer content of atleast 70%,

(2) a glycol component consisting essentially of

(a) about 95 to about 65 mol % 1,4-cyclohexanedimethanol, and

(b) about 5 to about 35 mol % poly(oxytetramethylene) glycol having amolecular weight of about 500 to about 1100, and

(B) about 2-98 weight % of a relatively rigid polyester having recurringunits from about 80-100 mol % terephthalic acid and at least onealiphatic or cycloaliphatic glycol having 2-12 carbon atoms, saidpolyester having an I.V. of about 0.5-1.0.

The blends have greater stiffness than the poly(etherester) alone. Theyalso have unexpectedly improved notched Izod impact strengths whencompared to the poly(etherester) alone when the blends contain about98-60 wt % of the flexible poly(estherester) and about 2-40 wt % of therelatively rigid polyester.

The dibasic acid component of the poly(etherester) consists essentiallyof 1,4-cyclohexanedicarboxylic acid having a trans isomer content of atleast 70%, preferably at least 80% and most preferably at least 85%trans isomer content.

The poly(oxytetramethylene) glycol component has a molecular weight ofbetween about 500 and about 1100, preferably about 1000 (weightaverage). It is used in an amount of from about 5 to about 35 mol %,preferably about 8-15%, based on the total glycol mol %.

The poly(etherester) further may comprise up to about 1.5 mol %, basedon the acid or glycol component, of a polybasic acid or polyhydricalcohol branching agent having at least three COOH or OH functionalgroups and from 3 to 60 carbon atoms. Esters of many such acids orpolyols may also be used. Suitable branching agents include trimelliticacid or anhydride, trimesic acid, trimethylol ethane and trimethylolpropane.

It should be understood that in this polymer the total acid reactantsshould be 100 mol %, and the total glycol reactants should be 100 mol %.Although the acid reactant is said to "consist essentially of"1,4-cyclohexanedicarboxylic acid, if the branching agent is a polybasicacid or anhydride, it will be calculated as part of the 100 mol % acid.Likewise, the glycol reactant is said to "consist essentially of"1,4-cyclohexanedimethanol and poly(oxytetramethylene) glycol, if thebranching agent is a polyol, it will be calculated as part of the 100mol % glycol.

The poly(etherester) compositions preferably contain a hindered,non-volatile phenolic antioxidant in an amount of from about 0.1 to 1.0,based on the weight of copolyesterether.

The poly(etherester) can be prepared by conventional techniques using anacid component consisting essentially of terephthalic acid. Minoramounts of other conventionally used dicarboxylic acids (up to about10%) such as isophthalic, naphthalene dicarboxylic or aliphaticdicarboxylic acids having about 4 to 12 carbon atoms may be used. Aglycol, or a combination of glycols for copolymers, having 2 to 12carbon atoms is used. The preferred glycols are ethylene glycol,1,4-butanediol and 1,4-cyclohexanedimethanol, and combinations thereof.Especially preferred glycols are ethylene glycol and1,4-cyclohexanedimethanol and combinations thereof. The I.V. of thepolyester is about 0.5-1.0.

The flexible poly(etherester) and the more rigid polyester can beblended by conventional plastic melt-mixing methods such as meltblending using an extruder.

The following are examples of components of highly flexible polymerblends which are useful in forming carrier films. Polymer "X" is apoly(etherester) having an I.V. of 1.23 and recurring units from

99.5 mol % 1,4-cyclohexanedicarboxylic acid

(trans content about 90%)

0.5 mol % trimellitic anhydride

91.1 mol % 1,4-cyclohexanedimethanol

8.9 mol % poly(oxytetramethylene) glycol

having a molecular weight of 1000

Polymer "Y" is an amorphous, linear polyester having an I.V. of 0.75 andrecurring units from

100 mol % terephthalic acid

68 mol % 1,4-cyclohexanedimethanol

32 mol % ethylene glycol

Polymer "Z" is an amorphous, linear polyester having an I.V. of 0.75 andrecurring units from

100 mol % terephthalic acid

69 mol % ethylene glycol

31 mol % 1,4-cyclohexanedimethanol

In forming specific carrier film blends, polymer X can be melt blendedwith polymer Y or polymer Z in various ratios to provide a carrier filmblend having the desired physical properties.

The present invention is based on the discovery that a certain aqueouspolyurethane composition is especially effective as a tie coat forimproving the adhesion of aqueous polyurethane paint compositions topolyester carrier films of the types discussed above. The paintcompositions are water-based polyurethane paints which are designed asautomotive finishes and which have heat softening and tensile elongationproperties that are compatible with those of the stretchable carrierfilm.

Aqueous polyurethane paint compositions having the desired propertiescontain polyurethanes which can be prepared by reacting an organicpolyisocyanate with an active hydrogen-containing compound such as apolyol or a polyamine or a mixture thereof to form a polyurethane, apolyurea or a mixed poly(urethane-urea). With regard to the paintcomposition, the term polyurethane is used broadly herein to mean any ofsuch polymers.

The organic polyisocyanate of the paint composition can be an aliphaticor an aromatic polyisocyanate or a mixture of the two. Diisocyanates arepreferred, although higher polyisocyanates can be used in place of or incombination with diisocyanates.

Examples of suitable aromatic diisocyanates are 4,4'-diphenylmethanediisocyanate and toluene diisocyanate. Examples of suitable aliphaticdiisocyanates are straight chain aliphatic diisocyanates such as1,6-hexamethylene diisocyanate. Also, cycloaliphatic diisocyanates canbe employed. Examples include isophorone diisocyanate and4,4'-methylene-bis(cyclohexylisocyanate). Examples of suitable higherpolyisocyanates are 1,2,4-benzene triisocyanate and polymethylenepolyphenyl isocyanate.

Water-based polyurethane basecoats in color-plus-clear compositions aredisclosed in U.S. Pat. No. 4,719,132. The polyurethane basecoatcompositions described in the patent can be used in the practice of thepresent invention. Other examples of useful water-based polyurethanesare disclosed in U.S. Pat. No. 4,149,679.

The aqueous polyurethane paint compositions can contain variousadditives, especially colorants. These include inorganic pigments suchas titanium dioxide, iron oxide, chromium oxide, lead chromate, carbonblack, silica, talc, china clay, metallic oxides, silicates, chromates,etc., and organic pigments, such as phthalocyanine blue, phthalocyaninegreen, carbazole violet, anthrapyrimidine yellow, flavanthrone yellow,isoindoline yellow, indanthrone blue, quinacridone violet, perylenereds, diazo red and others known to those skilled in the art.

The paints can also contain reflective particles in addition to acolorant and binder. Any particles conventionally employed in paints,particularly those employed in automotive finish paints can be used.Suitable reflective particles include metallic flakes such as aluminumflake, copper flake, bronze flake, copper bronze flake, nickel flake,zinc flake, magnesium flake, silver flake, gold flake, platinum flakeand other platelets such as mica, glass, stainless steel, coated mica,coated glass, and aluminum coated polyester film fragments. Mixtures oftwo or more types of particles can be used. It is believed that paintscontaining a substantial concentration of such pigments and reflectiveparticles present even greater adhesion problems on polyester carrierfilms. The tie layers of the present invention contribute to thesuccessful adhesion of such paint compositions.

The paint layer preferably has a mean thickness in the range of fromabout 0.012 to 0.080 millimeters. A particularly preferred meanthickness is in the range of from about 0.020 to 0.060 millimeters.

Examples of useful aqueous polyurethane paint compositions are listed inTables I, II and III.

                  TABLE I                                                         ______________________________________                                        Ingredient           Approximate % Weight                                     ______________________________________                                        Deionized Water      50                                                       Urethane Resin       25                                                       Aluminum Paste       5                                                        Ethylene Glycol Monohexyl Ether                                                                    5                                                        N-Methylpyrrolidinone                                                                              5                                                        Diethylene Glycol Monobutyl Ether                                                                  1                                                        N,N-Dimethyl Ethanolamine                                                                          1                                                        Xylene               1                                                        Aliphatic Solvent Naptha                                                                           1                                                        Isopropyl Alcohol    <1                                                       ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Ingredient           Approximate % Weight                                     ______________________________________                                        Deionized water      55                                                       Urethane resin       20                                                       Ethylene Glycol Monohexyl Ether                                                                    5                                                        N-Methylpyrrolidinone                                                                              5                                                        Diethylene Glycol Monobutyl Ether                                                                  1                                                        N,N-Dimethyl ethanolamine                                                                          1                                                        Titanium Dioxide/Mica                                                                              <1                                                       Silica               <1                                                       Carbon Black         <1                                                       Isopropyl Alcohol    <1                                                       ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Ingredient           Approximate % Weight                                     ______________________________________                                        Titanium Dioxide     25                                                       Ethylene Glycol Monohexyl ether                                                                    5                                                        Diethylene Glycol Monobutyl Ether                                                                  1                                                        Deionized Water      45                                                       N,N-Dimethyl Ethanolamine                                                                          1                                                        N-Methylpyrrolidinone                                                                              5                                                        Urethane Resin       20                                                       ______________________________________                                    

Applicants have discovered that the adhesion of an aqueous polyurethanepaint composition and a polyester carrier film of the types describedcan be markedly improved by using as a tie coat a self-crosslinking,aqueous polyurethane dispersion which is curable at room temperature.Dispersions of this kind are described in detail in the patent to Dabiet al, U.S. Pat. No. 4,335,029 issued Jun. 15, 1982 which isincorporated by reference herein.

According to the Dabi et al patent, the self-crosslinking polyurethaneis derived from an NCO-containing prepolymer. To make the polyurethane,the prepolymer is chain extended with an aliphatic polyamine chainextender such as an alkylene diamine or triamine. Preferably, the chainextender is a triamine-containing polyamine such as a mixture of a loweralkylene diamine and a lower alkylene triamine. Especially preferred isa mixture of ethylene diamine and diethylene triamine. After chainextension, a dihydrazide end-capper such as adipic dihydrazide is addedto the dispersion. The resulting hydrazide end-capped prepolymer is thenpartially or completely N-methylolated with formaldehyde. Thepolyurethane is thus provided with internal crosslinking, curablehardening groups activated to self-condensation and crosslinking upondrying of the latex film on a substrate at room temperature.

The prepolymer for the polyurethane is a derivative of a polyesterpolyol prepared from aliphatic diols and an aliphatic dicarboxylic acid.The preferred prepolymer is derived from neopentyl glycol, hexanedioland adipic acid. The prepolymer is NCO-terminated and contains units inthe chain derived from melamine and pendant water-dispersing carboxylicsalt groups. To synthesize the prepolymer, the polyester polyol isreacted with melamine and an aliphatic diisocyanate such as4,4'-bis(isocyanatocyclohexyl) methane. Carboxylic salt groups areinserted in the chain by reaction with an alpha, alpha-dimethylol C₂₋₁₀alkanoic acid such as 2,2-dimethylol propionic acid followed byneutralization with a salt forming agent such as a tertiary amine, e.g.,triethylamine. Through the use of appropriate diluents and agitationduring preparation of the polyurethane, as described in the Dabi et alpatent, a latex dispersion is obtained. This can be further diluted withwater or solvents to obtain the optimum viscosity for coating the tiecoat composition on the carrier by laminar flow coating.

A specific and preferred example, which applicants believe from theirown analysis and from the disclosure of Dabi et al to be such a selfcrosslinking dispersion, is "Witcobond W-240" dispersion, a product ofWitco Corporation of Houston, Tex. 77245. Typical properties ofWitcobond W-240 dispersion are listed in Table IV.

                  TABLE IV                                                        ______________________________________                                        Appearance              Translucent                                           Solids, %               30                                                    Particle Charge         Anionic                                               Particle Size           Colloidal                                             pH at 25° C.     8.5                                                   Viscosity at 25° C. (77° F.)                                                            <75                                                   Brookfield LVF, cps                                                           Flash Point, Tag Closed Cup, °C.                                                               >100                                                  Fugitive Organic Volatiles, weight %                                                                  13.4                                                  Surface Tension, dynes/cm                                                                             54                                                    Density, lb/gal         8.724                                                 Specific Gravity at 25° C.                                                                     1.05                                                  Glass Transition Temperature (Tg), °C.                                                         -53                                                   Shear Stability         Excellent                                             Freeze-Thaw Stability (-18° C.)                                                                Excellent after                                                               6 cycles                                              Tensile Strength at Yield, psi                                                                        6000                                                  Ultimate Elongation, %  70                                                    Hydrolytic Stability    Excellent                                             ______________________________________                                    

The sheet material of the invention also includes a transparentcrosslinked polyurethane topcoat. For some uses of thermoformable paintcoated sheet material, a transparent topcoat or clearcoat is optional.For other uses, as for high quality automobile finishes, a clearcoat iseither necessary or at least highly preferred. It can provide improvedsmoothness, high gloss, hardness, durability, resistance to weatheringand abrasion and scratch resistance. It provides the benefits of abasecoat/clearcoat system as now widely used in automobile finishes.

In addition, the applicants have found that when a self-crosslinking tielayer as described herein is used, the coating of a crosslinkedpolyurethane topcoat over the dried aqueous polyurethane paint layerfurther improves the adhesion of the paint layer to the tie-coatedcarrier film. Accordingly, the sheet materials of the invention have acrosslinked polyurethane transparent topcoat. This topcoat can becolorless, in which case it can be called a clearcoat, or it can belightly pigmented.

The crosslinkable polyurethane topcoat compositions used in makingapplicants' sheet material are known in the art. They are transparentand comprise (a) as one component, a crosslinkable polymer such as apolyester polyol, a polyurethane polyol or an acrylic polyol and (b) asa second component a polyisocyanate crosslinking agent. Each componentis dissolved in a volatile organic solvent such as acetone or toluene.The components are mixed just before the topcoat is coated over thepaint layer.

The dry thickness of the topcoat preferably is somewhat greater thanthat of the basecoat. It usually is in the range from about 0.02 to 0.2mm and preferably from about 0.03 to 0.1 mm. In view of its thickness,the topcoat can, if desired, be coated in two or more passes through thecoating process in order to provide layers that are thin enough for themost effective drying.

The sheet materials of the invention can be made with a wide range ofreactive, crosslinkable polyurethane topcoat compositions that areprepared for coating by mixing two or more reactive components beforecoating the mixture on a substrate. Examples of such compositionsinclude crosslinking polyurethane film-forming compositions which areformed by the reaction of a crosslinkable urethane polymer with acrosslinking agent. One such composition is the solvent-based systemcomprising a polyurethane polyol as one component and a polyisocyanateas the other as disclosed in the patent to Porter, U.S. Pat. No.4,719,132. Especially preferred is a two-component composition disclosedin this patent of which one component comprises a crosslinkablepoly(ester-urethane) polyol and the other comprises a polyisocyanatecrosslinking agent such as a polyfunctional aliphatic isocyanurate of1,6-hexamethylene diisocyanate.

A specific example of a preferred clearcoat composition is as follows:

                  TABLE V                                                         ______________________________________                                        Ingredient          Approximate % Weight                                      ______________________________________                                        A    Urethane Resin     60                                                         Toluene            30                                                         2-Butoxy Ethyl Acetate                                                                           10                                                         Benzotriazole       1                                                    B*   Polyfunctional aliphatic                                                                         100                                                        isocyanurate resin based on                                                   1,6-hexamethylene diisocyanate                                           ______________________________________                                         *Available as Desmodur N3300 from Mobay Corporation Coatings Division.   

The liquid tie coat, paint and topcoat compositions are coatedsuccessively on the carrier film, each coating being dried before anysubsequent coating is applied. The coating procedure employed is alaminar flow procedure as described in the Reafler patent applicationSer. No. 116,426. This includes direct extrusion onto the carrier filmby use of an extrusion hopper functioning to apply the coatingcomposition by bead coating or a stretch-flow coating mechanism. Anotherkind of laminar flow coating process involves extrusion of the coatingcomposition through a slot with subsequent travel down the slide surfaceof a slide hopper or subsequent free fall in a curtain coating process.Also included are curtain coating processes in which the curtain isgenerated by the use of an overflow weir. These coating processes canprovide a defect-free coating with a high degree of uniformity of layerthickness.

Extrusion hoppers are particularly effective devices for coating thepaint and tie coat compositions. Especially advantageous is the factthat such high precision equipment can coat very thin layers as isdesirable with tie layers. Such coating devices are also well-adaptedfor use with compositions which exhibit the shear-thinningcharacteristics typical of the described paint compositions. In suchhoppers, transverse pressure gradients are minimized and almost allpressure drop across the slot is in the direction of coating.

Coating techniques and apparatus for making sheet materials of thepresent invention are used in the manufacture of photographic productsand are described in U.S. Pat. Nos. 2,681,294, 2,761,791, 2,975,754,3,206,323, 3,632,374, and 4,051,278.

After being coated onto the support, the coated layers are converted toa dry and tack-free state by drying, for example by conventionaltechniques or by the method of the application of Fronheiser et al Ser.No. 166,083 filed Mar. 9, 1988, now U.S. Pat. No. 4,872,270 issued Oct.10, 1989, at temperatures of 25 to 100 degrees C.

The tie layer preferably has a dry thickness less than about 0.0025 mmand most preferably less than about 0.001 mm. The thickness preferablyis much less than the thickness of the paint layer. For example, thepaint layer preferably has a thickness at least 8 times the thickness ofthe adhesion-promoting tie layer.

Such thin tie coats, when coated by laminar flow, are susceptible tonon-uniformities such as craters and pinholes, which are also referredto as repellencies. These are spots or areas on the coated film surfacewhich are significantly reduced in thickness or completely void ofcoating material. They are usually circular in shape. To eliminate orreduce the number of such craters and pinholes when very thin coats areapplied by laminar flow coating, it is desirable to include a surfactantin the tie coat composition. Especially desirable is the use of anon-ionic fluorinated surfactant of the formula ##STR1## wherein R=H orlower alkyl, m=1 to 4, n=5 to 9 and p=2 to 30, as disclosed in thecommonly-assigned copending application of applicants' co-workersHayward et al, Ser. No. 237,129 filed Aug. 29, 1988. The preferredsurfactant of this class is "FLUORAD FC 170C" surfactant supplied by 3MCompany.

Although good adhesion is obtained in the sheet materials of theinvention with or without a surfactant, the use of an appropriatesurfactant, especially one of the class described, reduces any tendencyof the tie coat composition to form craters and pinholes at low coatingcoverages. Thus, in a test disclosed in the copending application Ser.No. 237,129, the use of "Witcobond W-240" composition as a tie coatwithout an added surfactant resulted in irregular patterns or defectscaused by coating inconsistencies. Although such defects are acceptablefor some purposes, the preferred products of the invention aresubstantially free of them, as illustrated in Example 3 of the citedapplication.

The tie coat dispersion can also include minor amounts of other addendasuch as solvents, e.g., N-methylpyrrolidinone, diethylene glycolmonobutyl ether, ethylene glycol monohexyl ether, butyl acetate andpropylene glycol; thickeners such as polyacrylics and cellulosics;polymer modifiers such as acrylic latices; defoamers and crosslinkers.The polymer components and addenda are dispersed in water to yield adispersion which contains from about 60 to 95 weight percent water,preferably 70 to 90 weight percent water. The tie layer composition cancontain from about 0.001 to 0.50 weight percent of the non-ionicsurfactant, preferably 0.01 to 0.30 weight percent. The solids contentof the dispersion is in the range from about 5 to about 40 weightpercent.

The applicants' earliest work with the tie layer compositions describedherein indicated that the adhesion of the various layers of the sheetmaterial having a basecoat and a clearcoat over the tie coat wasimproved by subjecting the carrier film to corona discharge treatmentbefore coating the tie coat and the aqueous polyurethane paintcomposition on it. Later results, however, tend to show that theadhesion improvement is about equal when using the described tie layer,whether or not the layer is corona discharge treated. In any event,corona discharge can be used if desired and in some cases, may furtherimprove adhesion. Corona supplied by AC or DC current or a combinationof AC superimposed on DC can be used. However, AC corona is preferredsince a continuous wave AC corona requires less power. The coronadischarge treatment of films is well known, being described, forexample, in the patents to Kerr et al U.S. Pat. Nos. 3,531,314 andAmbusk 3,549,406 and further description of such treatments is believedunnecessary.

Although the sheet material of the present invention preferably isformed by laminar flow coating methods, improvement in adhesion of anaqueous polyurethane color coat to a polyester carrier film by using thetie coat composition described herein, can be achieved even with lessdesirable coating methods. Other such coating methods that have beenproposed for paint coated films include spraying and reverse rollcoating as described by, for example, the patent to Short et al, U.S.Pat. No. 4,769,100 (General Motors), European Patent Application No.285,071 A2, published Oct. 5, 1988 (Avery International Corp.); EuropeanPatent Application No. 266,109 A2 published May 4, 1988 (Rexham Corp.);and European Patent Application No. 230,364 published Jul. 29, 1987 (3MCompany).

The examples which follow provide further understanding of theinvention. The control examples describe unsuccessful attempts to adhereaqueous polyurethane paints to a polyester carrier film while theinvention examples illustrate preferred embodiments of the invention.

EXAMPLE 1

A stretchable polyester carrier film made by extruding a blend of alinear polyester and a rubbery copolymer as described in the patent toWeemes et al, U.S. Pat. No. 4,582,876 was coated by means of anextrusion hopper of the type disclosed in the patent to Beguin, U.S.Pat. No. 2,681,294 with a thin layer of Witcobond W-240 aqueouspolyurethane dispersion to serve as a tie layer at a dry coverage of0.54 g/sq m. After drying, the thickness of the tie layer wasapproximately 0.0005 mm. The dispersion contained 0.05 weight percent"Fluorad FC 170C" surfactant. Over the dried tie layer was then coatedat a dry coverage of 43 g/sq m an aqueous polyurethane paint layer ofthe composition listed in Table I. The paint layer was dried to athickness of about 0.038 mm. The paint coated film was thereafter coatedwith a two-component polyurethane clearcoat of the compositions listedin Table V and dried. Then a second clearcoat layer of the samecomposition and thickness was applied. After drying, the total clearcoatthickness was approximately 0.062 mm. The resulting sheet materialhaving a basecoat, tie coat and clearcoat was conditioned at 38° C. and100 percent RH (relative humidity) for more than four weeks and was thensubjected to adhesion tests.

EXAMPLE 2

The procedure of Example 1 was repeated except that before coating theWitcobond W-240 tie coat composition on the polyester carrier film, thefilm was corona discharge treated. The treatment consisted of exposingthe film to a 10 KHz, 10 Kv AC corona at a film speed of about 15m perminute for an exposure of about 8800 joules per m².

COMPARATIVE EXAMPLES

A and B:

The procedures of Examples 1 and 2, respectively, were repeated exceptthat no tie coat was applied to the carrier film.

C to F:

The procedures of Examples 1 and 2 were repeated with a series ofdifferent tie coat compositions, as indicated in Table VI below.

The film samples of the invention and of the comparative examples weresubjected to peel testing. The results are listed in Table VI. As thetable shows, the paint layers of certain samples were tested before theclearcoat was applied. Other samples were tested after application ofthe clearcoat.

Also as indicated in the table, clearcoated samples were tested with andwithout heat and humidity conditioning treatment. The latter consistedof maintaining the sample at 38° C. and 100% RH for more than four (4)weeks. The peel testing was done according to modification of ASTM testD-903, the samples being 15 mm wide. The peel angle was 180 degrees.Results are reported as the average and one standard deviation from themean.

                                      TABLE VI                                    __________________________________________________________________________                      Peel Strength g/in width                                            Tie       Fresh Fresh Paint                                                                          Conditioned                                    Example Coat   CDT                                                                              Paint Only                                                                          Plus Clear                                                                           Paint/Clear                                    __________________________________________________________________________    1       W-240  No 635 ± 39  >3000                                          2       W-240  Yes                                                                              745 ± 33                                                                         2048 ± 46                                                                         >3000                                          Comparative A                                                                         None   No 256 ± 41                                                                         2136 ± 92                                                                          930 ± 194                                  B       None   Yes                                                                              302 ± 29                                                                          1892 ± 368                                                                       1125 ± 133                                  C       PWSA 1000                                                                            No 874 ± 9                                                                          1912 ± 30                                                                         97 ± 7                                      D       PWSA 1000                                                                            Yes                                                                              918 ± 17                                                                         1905 ± 37                                                                          86 ± 20                                    E       W-260  Yes                                                                              94 ± 3                                                                            1810 ± 316                                                                        29 ± 10                                    F       Sancure 878                                                                          Yes                                                                              563 ± 32                                                                         1810 ± 47                                                                           3 ± 0.5                                   __________________________________________________________________________

In the above table:

"W-240" means "Witcobond W-240" self-crosslinking aqueous polyurethanedispersion as used in the sheet materials of the invention.

"PWSA 1000" is an acrylic urethane adhesion promoter supplied by PPGIndustries, Inc. and has the following ingredients:

    ______________________________________                                                            Approximate Weight                                        ______________________________________                                        Deionized water       75                                                      Acrylic Resin         10                                                      Urethane resin        10                                                      N-Methylpyrrolidone   1                                                       Diethylene Glycol Monohexyl Ether                                                                   <1                                                      Ethylene Glycol Monohexyl Etherl                                                                    1                                                       N,N-Dimethyl Ethanolamine                                                                           <1                                                      ______________________________________                                    

"W-260" means "Witcobond W-260" which is a semi-colloidal urethanedispersion supplied by Witco Corporation and which differs substantiallyfrom the W-240 colloidal dispersion.

"Sancure 878" is a non-crosslinking polyurethane dispersion differentfrom Witcobond W-240 dispersion and is supplied by Sanncor Industries,Inc., Leominster, Mass.

As Table VI shows, the sheet materials of the invention (Examples 1 and2) had excellent adhesion of the clearcoat and paint layers to thepolyester carrier film which was coated with a thin layer of WitcobondW-240 dispersion. The peel strength for the corona treated sample andfor the untreated sample were both much greater than 3000 g per inch ofwidth. Furthermore, the adhesion of the clearcoated sample was muchgreater than that of the sample which had only a paint layer over thetie coat. Also, the adhesion improved rather than decreased when theclearcoated sample was aged at 38° C. and 100% RH.

In comparison, samples A and B which had no tie coat showed loweradhesion and the adhesion decreased substantially when the samples wereaged at 38° C. and 100% RH. The other comparative examples showed pooradhesion after being conditioned.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention. We claim: 1. A stretchable, thermoformable protective anddecorative sheet material for use in a thermoforming process in which itis stretched and bonded to a three dimensional substrate which comprises

a flexible and stretchable, thermoformable polyester carrier film

a polyurethane paint layer formed from a water-based polyurethane paintcomposition,

a crosslinked polyurethane transparent topcoat, and

disposed between the carrier film and the paint layer, a thin tie layerformed by coating on the carrier film a self-crosslinking aqueouspolyurethane anionic colloidal dispersion, the polyurethane beingderived from a polyurethane prepolymer of which the chain is extendedwith an aliphatic polyamine chain extender and is end capped withN-methylol hydrazide groups, the prepolymer being derived from (a) analiphatic diisocyanate and (b) an aliphatic polyester polyol and havingpendant water-dispersing carboxylic salt groups in the polymer chain;said sheet material having a peel strength greater than 3000 g/in afterconditioning at 38° C. and 100 percent RH. 2. A sheet material accordingto claim 1 wherein the carrier film is formed from an immiscible blendof a linear polyester and a more flexible copolymer. 3. The sheetmaterial of claim 2 wherein the linear polyester is a copolymer ofterephthalic acid, 1,4-cyclohexanedimethanol and a lower alkylene glycoland the more flexible copolymer is a rubber-modified styrene-maleicanhydride copolymer. 4. The sheet material of claim 1 wherein theaqueous polyurethane dispersion from which the tie layer is formedcontains a surfactant. 5. The sheet material of claim 4 wherein thesurfactant is a non-ionic fluorinated surfactant. 6. The sheet materialof claim 5 wherein the surfactant is of the formula: ##STR2## whereinR=H or lower alkyl, m=1 to 4, n=5 to 9 and p=2 to 30. 7. A sheetmaterial according to claim 1 wherein the aliphatic polyester polyol isa derivative of neopentyl glycol, hexanediol and adipic acid and whereinthe prepolymer contains units derived from melamine. 8. A sheet materialaccording to claim 7 wherein the aqueous polyurethane dispersion fromwhich the tie layer is formed contains a non-ionic fluorinatedsurfactant and wherein the thickness of the tie layer after it is driedis less than about 0.0025 mm. 9. A sheet material according to claim 8wherein the thickness of the tie layer after it is dried is less thanabout 0.001 mm. 10. A sheet material according to claim 1 wherein thecarrier film is a corona discharge treated film. 11. An articlecomprising a three dimensional substrate having bonded thereto astretchable, thermoformable protective and decorative sheet materialcomprising a stretchable thermoformable polyester carrier film, apolyurethane paint layer formed from a water-based polyurethane paintcomposition, a crosslinked polyurethane paint composition, a crosslinkedpolyurethane transparent topcoat, and a thin tie layer formed by coatingon the carrier film a self-crosslinking aqueous polyurethane anioniccolloidal dispersion, the polyurethane being derived from a polyurethaneprepolymer of which the chain is extended with an aliphatic polyaminechain extender and is end capped with N-methylol hydrazide groups, theprepolymer being derived from (a) an aliphatic diisocyanate and (b) analiphatic polyester polyol and having pendant water-dispersingcarboxylic salt groups in the polymer chain; said sheet material havinga peel strength greater than 3000 g/in after conditioning at 38° C. and100 percent RH.

We claim:
 1. A stretchable, thermoformable protective and decorativesheet material for use in a thermoforming process in which it isstretched and bonded to a three dimensional substrate which comprisesaflexible and stretchable, thermoformable polyester carrier film apolyurethane paint layer formed from a water-based polyurethane paintcomposition, a crosslinked polyurethane transparent topcoat, anddisposed between the carrier film and the paint layer, a thin tie layerformed by coating on the carrier film a self-crosslinking aqueouspolyurethane anionic colloidal dispersion, the polyurethane beingderived from a polyurethane prepolymer of which the chain is extendedwith an aliphatic polyamine chain extender and is end capped withN-methylol hydrazide groups, the prepolymer being derived from (a) analiphatic diisocyanate and (b) an aliphatic polyester polyol and havingpendant water-dispersing carboxylic salt groups in the polymer chain;said sheet material having a peel strength greater than 3000 g/in afterconditioning at 38° C. and 100 percent RH.
 2. A sheet material accordingto claim 1 wherein the carrier film is formed from an immiscible blendof a linear polyester and a more flexible copolymer.
 3. The sheetmaterial of claim 2 wherein the linear polyester is a copolymer ofterephthalic acid, 1,4-cyclohexanedimethanol and a lower alkylene glycoland the more flexible copolymer is a rubber-modified styrene-maleicanhydride copolymer.
 4. The sheet material of claim 1 wherein theaqueous polyurethane dispersion from which the tie layer is formedcontains a surfactant.
 5. The sheet material of claim 4 wherein thesurfactant is a non-ionic fluorinated surfactant.
 6. The sheet materialof claim 5 wherein the surfactant is of the formula: ##STR3## whereinR=H or lower alkyl, m=1 to 4, n=5 to 9 and p=2 to
 30. 7. A sheetmaterial according to claim 1 wherein the aliphatic polyester polyol isa derivative of neopentyl glycol, hexanediol and adipic acid and whereinthe prepolymer contains units derived from melamine.
 8. A sheet materialaccording to claim 7 wherein the aqueous polyurethane dispersion fromwhich the tie layer is formed contains a non-ionic fluorinatedsurfactant and wherein the thickness of the tie layer after it is driedis less than about 0.0025 mm.
 9. A sheet material according to claim 8wherein the thickness of the tie layer after it is dried is less thanabout 0.001 mm.
 10. A sheet material according to claim 1 wherein thecarrier film is a corona discharge treated film.
 11. An articlecomprising a three dimensional substrate having bonded thereto astretchable, thermoformable protective and decorative sheet materialcomprising a stretchable thermoformable polyester carrier film, apolyurethane paint layer formed from a water-based polyurethane paintcomposition, a crosslinked polyurethane paint composition, a crosslinkedpolyurethane transparent topcoat, and a thin tie layer formed by coatingon the carrier film a self-crosslinking aqueous polyurethane anioniccolloidal dispersion, the polyurethane being derived from a polyurethaneprepolymer of which the chain is extended with an aliphatic polyaminechain extender and is end capped with N-methylol hydrazide groups, theprepolymer being derived from (a) an aliphatic diisocyanate and (b) analiphatic polyester polyol and having pendant water-dispersingcarboxylic salt groups in the polymer chain; said sheet material havinga peel strength greater than 3000 g/in after conditioning at 38° C. and100 percent RH.